It has been proposed that 1-aminocyclopropane-1-carboxylic acid (ACC) oxida
se catalyzes the oxidation of ACC to ethylene via N-hydroxyl-ACC as an inte
rmediate, However, due to its chemical instability the putative intermediat
e has never been isolated. Here, we have shown that a purified recombinant
ACC oxidase can utilize alpha -aminoisobutyric acid (AIB), an analog of ACC
, as an alternative substrate, converting AIB into CO2, acetone, and ammoni
a. We chemically synthesized the putative intermediate compound, N-hydroxyl
-AIB (HAIB), and tested whether it serves as an intermediate in the oxidati
on of AIB. When [1-C-14]AIB was incubated with ACC oxidase in the presence
of excess unlabeled HAIB as a trap, no labeled HAIB was detected. By compar
ing the acetone production rates employing HAIB and AIB as substrates, the
conversion of HAIB to acetone was found to be much slower than that of usin
g AIB as substrate. Based on these observations, we conclude that ACC oxida
se does not catalyze via the N-hydroxylation of its amino acid substrate. A
CC oxidase also catalyzes the oxidation of other amino acids, with preferen
ce for the D-enantiomers, indicating a stereoselectivity of the enzyme. (C)
2001 Academic Press.